A seamless steel pipe, which is more reliable than a welded pipe is frequently used in a severe oil well or gas well (hereinafter collectively referred to as “oil well”) environment or in high temperature environment, and the enhancement of strength, improvement of toughness and improvement in sour resistance are therefore consistently required. Particularly, in oil wells to be developed in the future, the enhancement in strength and improvement in toughness of the steel pipe are needed more than ever before because a high-depth well will become the mainstream, and a seamless steel pipe also having sulfide stress cracking resistance (hereinafter “SSC resistance” for short) is increasingly required because the pipe is used in a severe corrosive environment.
The hardness, namely the dislocation density, of a steel product rises as the strength is enhanced, and the amount of hydrogen which penetrates into the steel product increases to make the steel product fragile to stress because of the high dislocation density. Accordingly, the SSC resistance generally deteriorates against the enhancement in the strength of the steel product which is used in a hydrogen sulfide-rich environment. Particularly, when a member which has the desired yield strength is produced by use of a steel product with a low ratio of “yield strength/tensile strength” (hereinafter referred to as yield ratio), the tensile strength and hardness are apt to increase, and the SSC resistance remarkably deteriorates. Therefore, when the strength of the steel product is raised, it is important to increase the yield ratio in order to keep the hardness low.
Although it is preferable to make the steel product into a uniform tempered martensitic microstructure in order to increase the yield ratio, that alone is not sufficient. One method for further enhancing the yield ratio in the tempered martensitic microstructure is the refinement of prior-austenite grains (hereinafter referred to merely as “austenite grains”). The said refinement of austenite grains is also effective in increasing the toughness of a high strength steel product.
However, the refinement of austenite grains needs an off-line quenching treatment, which deteriorates the production efficiency and increases the energy used. Therefore, currently this method is disadvantageous due to the rationalization of cost, improvement in production efficiency and energy saving which are all indispensable to manufacturers.
Thus, some technologies for the refinement of austenite grains by adding Nb, in a production process including a highly productive in-line quenching treatment, are disclosed in the Patent Documents 1 to 3. Further, a technology for the refinement of austenite grains by controlling the contents of N and Nb, in a production process including an in-line quenching treatment, is disclosed in the Patent Document 4.    Patent Document 1: Japanese Laid-open Patent Publication No. 05-271772,    Patent Document 2: Japanese Laid-open Patent Publication No. 08-311551,    Patent Document 3: Japanese Laid-open Patent Publication No. 2000-219914    Patent Document 4: Japanese Laid-open Patent Publication No. 2001-11568